Problem: Solve for $a$ : $a - 21 = 24$
Answer: Add $21$ to both sides: $ \begin{eqnarray} \\ a - 21 &=& 24 \\ \\ {+21} && {+21} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{-21 + a} &\hphantom{=}& \hphantom{24} \\ a &=& 24 {+ 21} \\ \end{eqnarray} $ Simplify. $ a = 45$